In medieval times there was a belief that God had created the Garden of Eden with plants that would remedy all humankind's woes: for every disease or desire, there was a botanic specimen that would salve or save. Taken one step further, the belief led to the doctrine of signatures which stated that each plant had a God-given sign as to its purpose. Hence foxgloves, now known as effective in heart conditions, were applied to lung problems - the spotted flowers supposedly being reminiscent of diseased lungs. (In George Eliot's Silas Marner, the hero correctly supplies the flower to an old cobbler's wife who has dropsy - congestive heart failure - but that was 1861, after science had sorted the truth out.)

David Stuart obviously regards some modern herbal cures, with their reassuring packaging and claims for no side effects, as pretty much an extension of that flawed earlier thinking. His thesis is that the number of plants that actually do effect our body chemistry are only a fraction of the 10,000 or so species that have been used by people, and this fraction contains some that are far more complex and misunderstood than those packets of optimistic natural remedies ever admit.

Hence he can say for certain that the Scottish presbyterians who eschewed tomatoes on the grounds that they were sexually arousing were mistaken (allowing, of course, for the placebo effect). But so are those who quaff herbal remedies thinking "natural" is synonymous with "harmless". To illustrate this, Stuart gives the interesting example of nux vomica, which has been revived recently as part of a "natural Viagra".

Long known in the east, the so-called "vomiting nuts" were described by the great English apothecary Gerard in the late 16th century. When the tree that produced the nuts was later found in India, it became the fashionable choice for disposing of inconvenient and unwanted husbands. In Rome, clandestine poisoning schools were even set up where abused wives might learn how to administer tiny, unnoticed doses to their spouses. These tiny amounts collected in the body and eventually led to a horrifying death from asphyxia and convulsive fits. In the words of one 19th-century account: "The abdominal muscles are as hard as a board, the chest is fixed, the face becomes livid, the eyeballs staring... Consciousness is retained to the last."

Italy and France had celebrated cases involving the aristocracy; in Britain there was the sensational career of Thomas Griffiths Wainewright, friend of Dickens and Blake. He first gave nux vomica to his wealthy uncle, inheriting a substantial fortune. Once that was spent, he set about his in-laws, killing two before anyone became suspicious. Wainewright fled to France, but later attempted to return and was captured. Astonishingly he avoided the gallows and was transported to Tasmania instead.

It was in the United States that nux vomica began to move out of the poisoner's cabinet and into the mainstream. Doctors prescribed minute doses for breathing difficulties, menstrual problems, labour pains and constipation. In this they were following earlier apothecaries who had a fascination for the more deadly plants available, usually assigning them astonishingly wide-ranging powers. Plague, for example, was treated by Nicholas Culpeper with the dangerous hedgerow flower, valerian; Gerard advised the more deadly yellow monkshood, which, as Stuart points out, probably helped a few sufferers to avoid the final grisly stages but cured no one. As for nux vomica, however, a new combination of the nuts with testosterone and the African rainforest tree bark, yohimbine, is being sold as a resurrection for the erection. The active constituent of nux vomica is strychnine.

At this point one wishes for some deeper delvings: Stuart suggests that the medieval fascination with serious poisons as remedies led to smaller and smaller doses, leading eventually to the principle of homeopathy where the poison may be almost or entirely absent. I would have liked to hear the homeopathist side of the argument, plus the evidence of any scientific studies, but to be fair, Stuart's reluctance to venture into such areas is also a reluctance to stray far from the botany - and that thematic stem is the volume's strength.

The nux vomica story, one of many in this fascinating work, illustrates the complexities surrounding human relationships with medicinal, psychoactive and just plain deadly plants. Not only is there often a vast jungly hinterland of legend, mystery and folklore, but science often weighs in with conflicting or inconclusive evidence. Add to that a thicket of moral, religious and legal problems, plus of course the placebo effect, and the subject can appear impenetrable. As a botanist and the author of several books on the history of the subject, Stuart wields a mean pair of shears as he finds us a logical route through the maze. And under each plant specimen he finds a goldmine of wonderful stories, giving us a lesson in the history of medical botany along the way (40% of modern drugs are derived from plants). There are nuggets here worthy of further investigation: Queen Henrietta Maria's doctor prescribing coca leaves, the Seven Singing Sutherland Sisters whose nine-foot long hair allowed their father to make a fortune from a hair tonic (concocted long after the hair had grown), and the fact that the Native American Church of the US is legally allowed hallucinogenic drugs that are barred from the rest of the population. In describing controversial plants such as cannabis or the Yemeni qat tree, Stuart's approach is balanced and unprejudiced - virtues sadly lacking in many quarters. The mind-boggling botany of Mexico is presented succinctly and with all the vivid colour that some of the plants can engender when eaten.

This is a fine book, beautifully illustrated too. And if it does not grapple with all of the issues it raises, we should at least be grateful that it raises them at all. As I finished reading, wishing it might have been longer, I remembered that wonderful Jan Brueghel painting, depicting the Garden of Eden with all the trees simultaneously laden with fruit and flowers while the leopards play next to the peacocks and deer. The Edenic myth that every problem has a plant to solve it dies very hard indeed, especially when new drugs like taxol appear on the market. Recently discovered to be a substance that prevents the division of breast cancer cells, it comes from the same yew tree oil once used by Scythian tribes to poison their arrows. Apparently, with Indian yew groves now stripped bare by worldwide demand, the hedge clippings of England are starting to fetch a decent price on the open market. Careful with those shears.